With higher integration of semiconductor devices, miniaturization of insulated gate field-effect transistors (MOS transistors) has been required for the purpose of inhibiting increase in the chip size.
To meet the requirement, gate electrodes each obtained by stacking a polysilicon film on a refractory metal film or a refractory metallic compound film (hereinafter referred to as a “metal-containing film”) are put in use. Here, the metal-containing film is used to inhibit a short channel effect which would occur due to the miniaturization, and accordingly to maintain a conventional threshold voltage. The polysilicon film is used to protect the metal-containing film, and to inhibit increase in the resistance of the gate electrode.
Nevertheless, in the case where RIE (Reactive Ion Etching) is used to etch a gate electrode film obtained by stacking the polysilicon film on the metal-containing film, there is a problem that a skirt-shape etching residue is likely to remain in a lower portion of the gate electrode film after the RIE process because of the three-dimensional structure of the gate electrode.
In addition, the problem that the gate electrode film is formed into a skirt shape or a tapered shape also occurs depending on a type of metal used for the gate electrode film, due to the lower vapor pressure of reaction products of an etching gas with the metal-containing film.
As a result, gate electrodes whose shapes are perpendicular to the semiconductor substrate cannot be obtained, and the gate electrodes vary in shape from one to another. Such variation deteriorates the gate characteristics, thereby leading to a problem of not obtaining MOS transistors having stable characteristics.
The foregoing problems further cause another problem that miniaturization of MOS transistors is so hindered as to make it difficult to inhibit the increase in the chip size.
On the other hand, methods of manufacturing a MOS transistor having a gate electrode using a refractory metal have been known. One of the semiconductor device manufacturing methods of such kind is disclosed in Japanese Patent Application Publication No. 6-53495.
The above method of manufacturing a semiconductor device includes the steps of; depositing a first layer of relatively high density refractory metal on a gate oxide layer formed on a silicon substrate; depositing a second layer of relatively low density refractory metal on the first layer of relatively high density refractory metal; etching the second layer to form an upper gate electrode portion; and etching the first layer to form a lower portion of the gate electrode that extends laterally beyond the upper gate electrode portion.
Nevertheless, the above method of manufacturing a semiconductor device is intended to form an inverted-T-shaped gate electrode, and provides neither disclosure nor suggestion about the above-described problem.